1. Field of the Invention
The present invention relates to driving method and apparatus to activate a light emitting device.
2. Description of the Related Art
Display units using a light emitting device are widely used in electronic devices of nowadays. In some cases, a desired display is achieved by changing the luminance of the light emitting device. There also are cases where luminous intensities of light emitting devices each having different shape and emitting area are adjusted to match with each other.
Conventionally, the luminance of a light emitting device is changed by using a plurality of power supplies each generating a different output level. Among plural outputs from those power supplies, outputs which can be used to produce desired luminance are selected by using switches and resulting outputs are summed to drive a light emitting device.
Alternatively, a light emitting device is activated by a pulse-width modulated output which, correspondingly to a desired luminance level, assumes an output level of a power supply, that generates a specified output, for a period corresponding to any number from 0 through k of divided sections obtained by dividing a luminous period of a light emitting device by "k".
As described above, in conventional cases, the luminance of a light emitting device is changed by using two or more power supplies numbers of which is determined to be an n-th power (n being an integer) of 2 as usually seen in the design of D/A converters and the outputs which together produce desired luminance are summed to activate a light emitting device. The pulse-width modulated signal is also used for activating a light emitting device as explained above.
Therefore, in the former example, as the number of steps of the luminous intensity is increased, more numbers of power supplies are required to generate such different outputs accordingly. In the latter example, as the number of steps of luminance intensity is increased, the length of time which can be allocated to each divided section becomes shorter. This, as a result, requires a light emitting device which can operate at a high speed so that light emission is possible even in such a short time.